THE RHYTHMIC NATURE OF LIFE 299 



metabolism in all animals and plants studied are different for the two 

 fortnights of a month. 



Whenever a biologist discovers some strikingly new phenomenon, he 

 immedately seeks ways by which to assure himself that the phenomenon 

 is a real one, and not the result of some error arising in the pecularities of 

 his apparatus or methods of analysis. This is especially true when one deals 

 with respirometers and rates of Oo-consumption. 



It was gratifying, therefore, when it was discovered that by simply 

 attaching threads to oysters and clams and measuring their tendency to 

 open their shells and hence ventilating their gills, one could find striking 

 daily and lunar rhythms resembling even in some detail the daily and lunar 

 rhythms of 02-consumption in the fiddler crabs, seaweed, salamanders, and 

 potatoes (Brown, Bennett, Webb, and Ralph, 1956; Brown, 1954b). 

 And when a rat was placed in an apparatus to measure its spontaneous 

 running activity and under conditions to randomize its normal overt daily 

 running period, the same form of daily rhythm was found again. The rat 

 resembled the earthworm, however, in showing greatest activity in the 

 early morning and early evening, instead of lowest at these times as in the 

 majority of the other species studied. The rat also possessed a lunar-day 

 cycle remarkably similar to that of the potato, and also 27-day and synodic 

 monthly cycles. 



Fiddler crabs, placed in automatic recorders of running activity, con- 

 tinue to exhibit at least for many days extremely conspicuous overt tidal 

 cycles of running. It is interesting to note here that the fiddler crab has, 

 thus, two simultaneous cycles of two frequencies, lunar and solar ; one is 

 the predominant regulator of activity and the other of color change. 



It seems clear, from a consideration of the information I have described, 

 that cycles of daily and lunar frequencies are continuously imposed upon 

 living things from the physical world. But you will recall that the daily 

 cycles of color change in the fiddler crab in the darkroom could be reset to 

 any hour of the day and would then persist indefinitely. Therefore there 

 could be no direct, simple relationship between the signals coming from 

 the external environment and what the crab w^as doing at any given time. 

 The crab must have also an internal clock of some kind. Some brief experi- 

 ments have given us reason to believe that, when one resets the time of the 

 color-change clock, the imposed Oo-consumption clock is not reset, but 

 continues responding directly to the outside factors. The color-change 

 clock appears capable of being set in any relationship to the metabolic 

 clock. This may perhaps be compared to the factory worker who, in chang- 

 ing his time of work from the day to the night shift, does so without re- 

 setting his watch. 



We have been able to separate to some extent the "inner" and "outer" 



